The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
The spine is a flexible column formed of a plurality of bones called vertebrae. The vertebrae include a hollow cavity and essentially stack one upon the other, forming a strong column for support of the cranium and trunk of the body. The hollow core of the spine houses and protects the nerves of the spinal cord. The different vertebrae are connected to one another by means of articular processes and intervertebral, fibrocartilaginous bodies. The intervertebral bodies, also known as intervertebral disks, include a fibrous ring filled with pulpy material. The disks function as spinal shock absorbers and also cooperate with synovial joints to facilitate movement and maintain flexibility of the spine. When one or more disks degenerate through accident or disease, nerves passing near the affected area may be compressed and consequently irritated. The result may be chronic and/or debilitating neck and/or back pain due to these spinal disorders.
One procedure for treating spinal disorders involves using substantially rigid plates for fixation of two or more vertebrae in desired spatial relationships and orientations relative to each other. During the procedure, the spine can be approached anteriorly or posteriorly. In either case, holes are drilled and tapped in at least two of the vertebrae to receive screws or other fasteners that secure the plate. The holes are positioned with reference to apertures formed in the plate. Typically the plate is curved about its longitudinal axis to facilitate contiguous surface engagement of the plate with the vertebrae. With the plate maintained against the vertebrae, the fasteners are driven into the vertebrae through the apertures in the plate. As a result, the plate maintains the attached vertebrae in a desired spacing and orientation with respect to each other.
Over time, some fasteners may gradually work loose from the vertebrae. Slight shock or vibration of the vertebrae, due to walking, climbing stairs or more vigorous activity by the patient following treatment increases this tendency, jeopardizing the integrity of fixation. Moreover, as the fasteners work loose, the outward protrusion of the heads over other components of the fasteners can be a source of discomfort and present the risk of trauma to adjacent and surrounding soft tissue. Some plates include a retention mechanism that prevents the screws from working loose after fixation.
Occasionally, the fasteners may not be inserted at a proper insertion angle during the fixation procedure. When the fastener is inserted at an improper angle, the retention mechanism may not be able to contact the fastener as the fastener backs away from the vertebra. The retention mechanism may increase the complexity of manufacturing and assembly. For example, the spinal plate may require features such as additional openings in the plate to assemble the retention mechanism. These openings may decrease the structural integrity of the plate. The retention mechanism may require various features that interact with springs or other compression members that bias the retention mechanism in one or more directions. Additional features such as stops that prevent the retention mechanism from moving too far relative to the plate or from over-compressing the springs may also complicate manufacture and assembly.